I, um, bow down to entropie's mastery of the art
The coolest thing I've written is a backdoor program for Linux systems which locks all its pages in RAM, runs with realtime priority and gives you a (basic-cleartext-transmitted-password-security-protected) custom shell prompt on the system with its own rudimentary command language
I thought it was a cool app. Saved us a few times from having to call our datacenter to reboot a misbehaving server.